DE 32 40 404 A1 discloses a braking system with slip control for automotive vehicles which comprises a brake force booster coupled to the tandem master cylinder. Located between the tandem master cylinder and the hydraulic brake force booster, within the housing of the brake pressure generator, is a positioning piston to which pressure from an external energy source is dynamically applied during the brake slip control through a corresponding valve and switching arrangement. In order to allow the manually controlled actuating force, the pressure rod and, hence, the use of the brake pedal is possible at any time during the slip control mode.
In the generic braking system according to EP-A-0374485 the master brake cylinder formed as an electrohydraulic follower booster is connected through a piston rod to a braking pedal. The piston rod applies pressure to a master cylinder piston through which, in turn, a braking chamber can be pressurized. The brake fluid is passed through brake conduits to the corresponding wheel brake cylinder. Associated to the master cylinder piston is a servo piston chamber which, through magnetic valves, is connected to a pressure fluid accumulator and a pump delivering into the pressure fluid accumulator. Moreover, a pressure limiting valve is provided in the bypass leading to the pump so that an auxiliary pressure source formed of the magnetic valves, the accumulator, a check valve and the pressure limiting valve with the pump delivers the pressure fluid of an intake reservoir in communication with the brake pressure generator either to the servo piston chamber or back to the intake reservoir. The magnetic valves of the auxiliary pressure source are controlled through a sensor detecting the movement of the piston rod. Moreover, a sensor is also associated to the master cylinder piston, detecting independently of the movement of the piston rod the actual movement of the master cylinder piston and also enabling the magnetic valves for the control of the auxiliary pressure source to be actuated by means of an electronic assembly. The switching and mechanical efforts required for providing the auxiliary pressure in the brake pressure generator are disadvantageous. Defective magnetic valves of the auxiliary pressure source result in a failure of the servo pressure for the brake pressure generator.
The switching principle of the braking system for controlling the brake slip is to be improved with a view to the multiplicity of required inlet and outlet valves. In special operating modes of the braking system, the expansion of the pressure fluid will have to take place in the non-pressurized intake reservoir which is likely to result in operating failure. In addition, the structural design of the brake pressure generator does not permit a servo force control characteristic that can be performed independently of the manual input force to allow, if need be, a simple change in the brake characteristic. Also, the use of a relatively complex auxiliary pressure generator (motor pump aggregate) proves to be cost-intensive.
It is, therefore, the object of the invention to provide a braking system with slip control in which the afore-mentioned disadvantages have been overcome, permitting versatile use through a simple, low-cost and safe design, which can be extended to multi-channel operation involving low energy consumption. Another object of the invention resides in enabling the brake force booster function to be varied within predetermined limits independently of the pressure rod actuation.
These objects are achieved providing an electronic sensor means for detecting the relative movement between the pressure rod and the booster piston, the output signals of which are able to influence the excitation of the auxiliary pressure source. Hence, the brake force boosting is effected, if need be, independently of actuating forces put in manually, thereby enabling at any time by simple means an anti-locking control/traction slip control function.